1. Field of the Invention
This invention relates to a method for metallization in the preparation of semiconductor devices. More particularly, it relates to a method for aluminum metallization for uniformly burying an aluminum-based material in a contact section having a barrier metal structure.
2. Description of Related Art
In keeping up with an increasing refinement of the design rule for semiconductor devices, as exemplified by VLSIs or ULSIs of recent origin, the contact hole formed in an interlayer insulating film for establishing electrical an connection between an upper metallization and a lower metallization is becoming increasingly smaller in diameter, such that the aspect ratio now exceeds unity. The upper metallization is usually formed by depositing an Al-based material by sputtering. However, it is difficult with this method to achieve sufficient step coverage to fill the contact hole having such a high aspect ratio so that a disconnection is frequently produced.
Recently, a high temperature sputtering has been proposed as a method for improving step coverage. This technology resides in carrying out sputtering with heating a wafer by a heater block or the like to several hundreds of degrees centigrade and applying an RF bias through the heater block, as introduced in, for example, Monthly Semiconductor World, December issue, 1989, pages 186 to 188, by Press Journal, or in IEEE/IRPS, 1989, pages 210 to 214. It is possible with this method to improve step coverage under Al reflow effects under elevated temperatures and ion impacts by the application of the bias voltage for forming the Al-based layer having a flattened surface. In these treatises, it is reported that, when a titanium (Ti) layer is provided as an underlying layer for the Al-based layer, the Ti layer contributes to surface migration of Al atoms to achieve excellent step coverage.
Meanwhile, the Ti layer provided as an underlying layer for the layer of the Al-based material is naturally expected to display the function as a barrier metal layer. However, the Ti layer, while being an excellent contact material from the viewpoint of achieving low-resistance ohmic contact, cannot achieve the function of the barrier metal, if used alone. The reason is that, even if the Ti layer is interposed by itself between the silicon (Si) substrate and the Al-based layer, the reaction between Si and Ti and the reaction between Ti and Al proceed simultaneously, so that Al spikes onto the Si substrate cannot be prevented from being produced. The conventional practice has been to adopt a two-layer barrier metal composed of, for example, a TiN layer superimposed on a Ti layer (Ti/TiN system). More recently, a two-layer barrier metal composed of a Ti layer and a TiON layer (Ti/TiON system), produced by introducing oxygen during deposition of the TiN layer, has been proposed with a view to improving the effects of preventing Al diffusion in the TiN grain boundary by oxygen segregation in the boundary.
However, when barrier metal such as Ti/TiON barrier metal, is previously formed in a contact section, an Al-based material cannot be filled uniformly in the contact hole if the Al-based material is to be deposited by high temperature bias sputtering. It is assumed that, in a wafer shown in FIG. 1, an interlayer insulating film 3 having a contact hole 4 is formed on a silicon substrate 1, in which an impurity diffusion layer 2 is formed previously so that the contact hole 4 is contiguous to the impurity diffusion region 2, and a Ti layer 5 and a TiON layer 6 are stacked step by step so as to overlie at least the contact hole 4 to provide a barrier metal layer 7. If it is attempted to deposit a layer 8 of an Al-based material on the wafer by high temperature bias sputtering, the material cannot be deposited or buried uniformly in the contact hole, because voids 9 tend to be produced. It is because Al in the course of high temperature sputtering is in an intermediate state between the liquid state and the solid state and highly sensitive to the morphology of the surface of the underlying layer. That is, the TiON layer 6 has a columnar crystal structure in which the crystals are oriented with the longitudinal direction thereof extending substantially orthogonally with respect to the film surface, so that the layer exhibits rough surface morphology and is inferior in wettability and reactivity with respect to the Al-based material. Our experiments indicated that the buried state of the Al-based material could not be improved when the deposition rate was lowered to about half the usual deposition rate for the purpose of promoting the reaction at the boundary between TiON and Al.